1. Field of the Invention
The present invention relates to a thermal head which is applicable to a thermal printer.
2. Description of the Related Art
Various kinds of thermal head are known which thermal head utilizes Joule heat obtained by applying electric current to a resistor element.
One of the known kinds of thermal head comprises a plurality of heat generative elements which are disposed in a line on which a printing medium such as an ink sheet and a medium to be printed such as a transfer paper (print paper) are arranged so that by selectively applying current signal to the heating elements so as to generate heat, a part of the ink sheet is molten or sublimed by the heat so that the ink of the sheet is transferred to the print paper surface, as disclosed in Japanese Patent Application Laying Open (KOKAI) No.53-87240.
The technique disclosed in the patent document is aiming at reducing the interval between dots formed by the heating elements.
Another kind of thermal head comprising the heating elements which is not publicly known and aiming at different points is proposed which is arranged as mentioned below.
That is, the proposed thermal head arrangement is based on a stand point that to change the recording density of one dot, it is better to arrange a plurality of heat concentrating portions instead of an even heat distribution over an entire area of the thermal head. For this purpose, a rectangular window is opened in the center of the heating element so that heat is concentrated to be generated at the four corners of the rectangular window which is shaped in a form which also enables to make the dot genuine circular as possible.
More precisely, an electrically insulating layer such as a glazed layer is formed on a substrate which is elongated along the direction X which is perpendicular to the direction Y to which the ink sheet and the print paper are fed and conveyed. A plurality of strip shaped heating resistor elements are disposed over the insulation layer covering the layer. On each of the heating resistor layers, a common electrode and an independent electrode are formed on both sides of the strip of the resistor facing together forming a predetermined gap between the electrodes to form a resistor element in the center portion of the resistor layer between the electrodes. Further, a rectangular window is formed open in the center of the heating element formed in the space between the electrodes.
In accordance with the structure of the thermal head mentioned above, at the time when the printing operation is conducted, heat is concentrated to be generated at four corners of the rectangular window.
There are two ink transfer modes, i.e., printing modes, for operating the thermal head. A first mode is to transfer ink in such a way that in a state wherein the ink sheet and the print paper which were conveyed in the direction Y along the strips of heating elements are stopped and being stationary, an electric current signal is selectively applied to the heating resistor elements so as to generate heat therefrom so that a part of the ink sheet is molten or sublimed so that the ink is transferred to the print paper surface.
A second mode is to transfer ink in such a way that in a state wherein both or one of the ink sheet and the print paper is being conveyed in the direction Y along the strip of heating resistor layer, an electric current signal is selectively applied to the heating resistor elements so as to generate heat therefrom so that a part of the ink sheet is molten or sublimed so that the ink is transferred to the print paper surface.
In the case where the first mode is to be adopted, to obtain a dot of genuine circle, the four sides of the rectangular window are arranged equal to each other. While in the case where the second mode is to be adopted, for the same reason, the sides of the window in the direction Y is shorter than that in the direction X.
However, in accordance with either of the first and second modes, an excessive heat concentration phenomenon occurs at the heat concentrating portions at the four corners of the window. The extent of the heat concentration depends on the difference of the resistance value of the resistor between the portion where the window is formed and the portion where the window is not formed, that is, the ratio of the width along the direction X along which the heating elements are arranged.
More precisely, in the temperature distribution on the print paper in a section thereof along a line in the direction X including the two corners adjacent together in the direction X, there are some portions where the temperature exceeds a critical temperature Tc at which the print paper is molten as a result of which the printing quality is degraded.
To cope with this problem, for example, to adjust the ratio of the width of the window in the direction X as mentioned before, the width (a) of the window in the direction X has to be shortened so that assuming that a&lt;c/2 wherein (c) represents the pitch of the heating resistor elements, the gap between the adjacent dots becomes too wide, which causes to form stripes of direction Y in the print paper.